Photometry and Radiometry
We are human beings living inside the zone of radiation. But we can not feel all the radiation with our eyes. Our eyes can detect only those radiation whose wavelengths are within range of 370 nm to 780 nm. This range is called visible range of wavelength. The radiation within this visible range of wavelength is termed as light. Hence, light is an electromagnetic radiation and light has a certain frequency range or wavelength range to be observed by our eyes. Each radiation has its own energy. Light has the energy to stimulate our eyes. The electromagnetic radiation whose wavelength is more than 780 nm is called infrared radiation and it does not stimulate our eyes but stimulates our body as heat. Again the electromagnetic radiation whose wavelength is less than 370 nm is called ultraviolet ray.
There are other radiations whose wavelengths are lesser than that of ultraviolet radiation such as radio wave, X-ray, etc. We also cannot observe these radiations as the wavelength of these radiations are less than 370 nm. Within the visible range of radiation, various wavelengths hold the various colors. The range 597 - 577 nm holds yellow which is in the middle of the visible wavelength range.
PhotometryPhotometry is a process of measuring light by correlating the visual sensation of a standard human observer. The standard viewer or standard human observer has a visual sensation which is the average of that of hundred numbers of visually fit people. It is already told that the healthy human eyes are sensitive to the visual range of wavelengths of electromagnetic waves. But it is also true that the human eyes are not equally sensitive to all wavelengths of electromagnetic waves within visual range. For some wavelengths, eyes are more sensitive and for some others, they are less sensitive.
Moreover, this sensitivity of eyes for the same wavelength of color can also vary with the intensity of light. That means the visual sensitivity of a color of the particular wavelength may be different in the bright and dim light. Depending on the brightness of the light there are three different types of human vision.
- Photopic Vision - Where high luminance levels adapt the eyes.
- Scotopic Vision - Where low luminance levels adapt the eyes.
- Mesopic Vision - Where intermediate levels of luminance adapt the eyes.
In the scopotic vision (vision in dim light), relation graph between visual sensitivity and wavelength of light is more or less similar to that in photopic vision but the peak of the curve is just shifted to wavelength 507 nm which corresponds to the bluish-green color. That means in the scotopic vision (vision in dim light) the human eyes have the maximum visual sensation to bluish-green color. The relation between visual sensitivity and wavelength in scotopic vision is expressed as another function V’(λ). Hence, the above graph shows two functions. V(λ) is for the Photopic vision (vision in bright light) and V’(λ) for Scotopic Vision (vision in dim light). Both these both functions allow us to derive the photometric quantity. Two graphs have a cross-sectional point at 555 nm. The color corresponding to this wavelength is equally sensitive for Photopic and Scotopic vision.